You need a configured weldment library. The default library isn't. How to create a configured weldment profile is mentioned in the weldment training material. I believe there is a download with a configured library. The new 2019 library is configured by default.
S-075348 When I create a structure system in the SOLIDWORKS® software, why are my weldment profiles unavailable?
Structure systems use only configured profiles.
In previous SOLIDWORKS® versions, the sample weldment profiles each contained one configuration. The folder structure of the weldment profile directory determined the standard, type, and size of each profile. In this setup, each SLDLFP file represented a size.
In the SOLIDWORKS 2014 version, it became possible to add multiple configurations to a weldment profile.
Effective with the release of SOLIDWORKS 2019, the sample weldment profiles each contain multiple configurations. The folder structure of the weldment profile directory determines the standard and type. In this setup, each SLDLFP file represents a type and contains multiple sizes.
Structure systems require the latter setup in which the folder structure determines the standard and type and the configuration determines the size.
To convert custom weldment profiles to the newer format:
1. Edit the SLDLFP file and add a configuration for each size.
2. Configure the sketch dimensions as necessary.
3. In Windows® Explorer, move the SLDLFP file up one level to the folder for the standard.
Structure systems are available effective with the SOLIDWORKS 2019 version.
It doesn't seem to have installed any configured weldments. Where would I be able to download the 2019 samples?